This invention is directed to a method of making a shadow mask for a color cathode ray tube. Color cathode ray tube shadow masks of the three-dimensionally curved type are conventionally made by a process in which a flat mask blank, typically 6 mil thick cold-rolled steel, is coated on both sides with a photosensitive, etchant-resistant material and then exposed from both sides through a registered pair of related mask masters. After exposure, the photoresist layers are developed and the blank is etched from both sides until a pattern of through apertures is formed in the mask blank. The blank is then shaped into the desired three-dimensional configuration and suitable processed for mounting in a color cathode ray tube.
As a result of the inevitable nonuniformities introduced during the process in which the flat mask blank is formed into a curved configuration, the end product shadow masks have mask aperture patterns which are not interregistrable -- that is to say, the variations in the locations of the mask apertures from mask to mask and the variations in mass aperture size, exceed maximum permissible tolerances.
As a result, it is standard practice today to use a finished shadow mask as the photographic stencil during the photo-deposition of a phosphor screen pattern. Since each phosphor screen is a unique replication of a particular shadow mask, and since all shadow masks are not interregistrable, it is necessary to "pair" or "mate" a given shadow mask with its uniquely associated phosphor screen throughout all factory operations on the tube, and to assemble the paired mask and screen at an appropriate stage of manufacture before the tube envelope is closed.
Because of the logistical problems which attend the pairing of masks and screens, and because of the cost associated with the afore-described pairing of masks and screens, it has long been a goal of color cathode ray tube manufacturers to develop a commercially practicable way to manufacture interregistrable shadow masks. In fact, the very first color cathode ray tubes manufactured did indeed have interregistrable masks. However, this was accomplished only by virtue of the fact that the masks and screens at the time were flat; the emanding tolerance constraints could thus be met. The resulting "flat pack" assembly of mask and screen soon became out-moded, performancewise, with the advent of the practice of screening the phosphor pattern directly on the concave inner surface of the color CRT faceplate. The use of curved screens and associated curved masks, however, rendered impracticable the making of interregistrable shadow masks.
The goal of finding an approach which would make possible the manufacture of interregistrable curved masks has remained an elusive one. U.S. Pat. No. 3,676,914-Fiore, discloses a method for making interregistrable three-dimensionally curved masks by which correlative flat mask masters are photographically projected onto photoresist layers on the convex and concave sides of a pre-curved shadow mask blank. The difficulties in accomplishing this flat-to-curve projection of the photographic master stencil patterns remains an obstacle to the perfection of this method of achieving interregistrability of curved masks.
A second approach, also noncommercial, to fabricating interregistrable curved masks is disclosed in U.S. Pat. No. 3,889,329 to Fazlin. There also, the mask blank is preformed before it is exposed from both sides. The Fazlin approach differs from the primary embodiment disclosed in the Fiore patent in that Fazlin's photographic mask masters are curved and are formed as part of a combined vacuum chuck and exposure lighthouse. The preformed blank is vacuum-clamped between the curved mask masters and is exposed to light sources located within vacuum chambers above and below the mask blank.
Yet another approach is achieving interregistrable curved shadow masks is to perform the mask blank into a curved shape, but rather than attempting to achieve the difficult step of exposing the convex side as well as the concave side of the curved blank, the photochemical etching of the mask is accomplished from the concave side only. A photographic master is used which is supported in close but nontouching relationship to the concave surface of the mask blank. This approach is described fully and claimed in the referent copending application. It is also disclosed, and aspects claimed, in U.S. Pat. Nos. 3,973,964 and 3,975,198, both assigned to the assignee of the present application.
Feasibility of the latter approach has been proven in the laboratory, however that approach, in its disclosed executions, employs an undesirably expensive mask blank material - a material having one one surface (ultimately the concave surface of the mask) a thin aperture-defining layer of nickel or other suitable material which is capable of withstanding the etching of mask apertures through the entire mask blank from the protected side only. In a preliminary operation, a pattern of apertures is etched in the aperture-defining layer. The underlying steel substrate is subsequently etched (through the apertures in the aperture-forming layer) using an etchant to which the aperture-defining layer is substantially resistant. Because of the highly competitive nature of the consumer television industry, the use of a mask material having a substantial price premium over simple cold-rolled steel stock may lessen somewhat the commercial attractiveness of the afore-described one-sided etch process.